In the related art, low melting point toner, i.e., toner having a melting point temperature that is lower than that of a conventional toner, containing crystalline polyester has a feature of quickly melting with heat. By using the low melting point toner, it is possible to fix a toner image on a sheet at a lower temperature of a fixing device.
In the fixing device, a contact-type thermistor (hereinafter, referred to as “thermistor”) is often used in order to control the temperature of the fixing device. The thermistor contacts a heat roller of the fixing device to measure temperature. For that reason, toner remaining on the heat roller after a fixing process may be attached to the thermistor. When conventional toner is used, some of the conventional toner that is attached to the thermistor during an image forming process is hardened and naturally detached from the thermistor. Alternately, the conventional toner attached to the thermistor is heated again and softened when subsequent fixing is performed. However, when the low melting point toner is used for image forming, it is found that a scratch is formed on a surface of the fixing roller.